1. Field of the Invention
The present invention relates to an improved drive system for large mining trucks and similar equipment. More specifically, the invention relates to the use of silicon controlled rectifiers (SCR's) in a diesel/electric propulsion system with an extended retarding circuit.
2. Description of the Prior Art
It is generally known and accepted commercial practice to employ a combined diesel/electric propulsion system in large mining trucks and similar heavy equipment applications wherein the propulsion system is also used to retard (dynamically brake) the vehicle. Typically, a diesel engine is used to drive an alternator which converts the mechanical energy to electrical energy. A rectifier assembly then converts the alternating current (AC) electrical output of the alternator to direct current (DC) output which in turn powers motorized wheels typically consisting of a DC motor coupled through a gear train to the driven wheel of the vehicle. Electronic regulators are provided to control excitation of the alternator and motor fields to produce the desired horsepower level and electrical mechanical switch gears (contactors) are used in the prior art to make the desired electrical circuit arrangement. For the purpose of retarding vehicle motion, power resistors for absorbing the electrical energy created when the motors in the motorized fields are operated as generators are provided. In this manner, the kinetic energy of the moving vehicle is converted to electrical energy and then dissipated as heat, thus slowing the vehicle. This is generally referred to as the "retarding" mode of operation which is used to control the speed of the vehicle and to slow down the vehicle to a speed that allows the conventional hydraulic brake system to then stop the vehicle.
It is also generally known and an accepted commercial practice to extend the application of the retarding torque by employing a series of electromechanical contactors in parallel with the retarding resistors such as to electrically shunt sections of the retarding resistors and thus, reduce the overall resistance as the vehicle slows down. In other words, as the vehicle slows down, the generator (wheel motor) voltage decreases as does the retarding current at fixed resistance which in turn means that the retarding torque decreases. To compensate for the continuing decrease in retarding torque, portions of the fixed resistance are sequentially shunted, thus periodically increasing the retarding current and torque until the speed of the vehicle is sufficiently controlled at low vehicle speed. Thus, these extended retarding contactors are necessary to provide high levels of retarding effort (torque) at low speeds.
Prior to the present invention, the implementation of extended retarding using electromechanicl contactors was generally limited by such practical considerations as a balance between benefit dervied versus increased capital costs and increased maintenance costs. For, it is generally known that energizing and de-energizing at high currents is highly destructive and as such, the contactor tips of the prior art are high maintenance items requiring periodic replacement. In contrast, the use of a solid state device such as a silicon controlled rectifier (SCR) may in principle solve the prior art problem of replacement of the contactor tips, but it is also generally known and accepted that SCR's are not to be employed as a switch in a DC circuit such as used in electrical motor retarding. The historical objection to the use of SCR's in retarding is two-fold. A SCR in the normal state blocks voltage applied in either direction, conducts in a forward direction when a pulse is applied to the gate and continues to conduct as long as current flows from anode to cathode. In other words, in a continuous DC circuit, the SCR does not turn off (open) unless the anode voltage supply is removed, reduced below the brake over voltage or reversed. Thus, the use of SCR's in the retarding circuit to shunt a portion of the power absorbing resistors is complicated by the very nature of how they are to be turned off and if turned off during retarding, the way that they are turned off implies a delay or relaxation period conceptually inconsistent with the purpose and intent of retarding (i.e., a pause in the braking torque).